(57) A radiation-shielding plastic material having an excellent transparency and superior
mechanical properties, is produced by polymerizing, in the presence of a lead compound,
a mixture of a vinyl monomer component (A) consisting of at least one member selected
from alkyl methacrylates and acrylates, hydroxyalkyl methacrylates and acrylates and
styrene and an organic acid component (B) containing, as an indispensable ingredient,
at least one member selected from the compounds of the formulae (I) through (IV):


 and
 wherein R1=C1-10 hydrocarbon radical, R2=C1-7hydrocarbon radical, R3 and R4=H or CH3, R5=C2-7hydrocarbon radical, R6=C1-3hydrocarbon radical and n=0~4.
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